1. Field of the Invention
The present invention relates to a capacitor used in a semiconductor device. More particularly, the present invention relates to an improvement of a dielectric film in a capacitor of a DRAM (Dynamic Random Access Memory).
2. Related Art
One of cells in a DRAM includes one transistor and one capacitor. The capacitor includes a lower electrode, a dielectric film, and an upper electrode. With the size reduction in DRAMs, the surface area required for the cells has been reduced, and hence it has been investigated to employ a three-dimensional electrode structure and a dielectric film having a high dielectric constant in order to obtain a given amount of capacitor capacitance in the limited area.
As for the dielectric film, Al2O3 has until recently been used as a dielectric film of a capacitor for DRAM, because Al2O3 has a dielectric constant of about 9, and because it is possible to obtain from Al2O3 a film having, in the same EOT (Equivalent Oxide Thickness), a lower leakage current as compared with a previously used silicon nitride film (dielectric constant=7), and as compared with an ON film (a laminated structure of silicon dioxide film and silicon nitride film) which is obtained by oxidizing the nitride film to form the oxide film (dielectric constant=3.9) on the nitride film.
With the reduction of the device size, however, it has to use a dielectric film having a higher dielectric constant. As a dielectric film in a capacitor for 70 nm level of DRAM, even hafnium oxide (HfO2) having a dielectric constant of 25 is not sufficient. Thus, in recent years, there has also been used a laminated film of Al2O3 and HfO2, or the like, which is somewhat inferior in the dielectric constant, but which has a larger band gap than HfO2 and is thereby capable of reducing leakage current in the same EOT. However, when such a film is used in an amorphous state, the effective dielectric constant of the film remains at about 20.
There has also been proposed a method in which yttrium (Y) is added to HfO2 to lower the crystallization temperature and to increase the dielectric constant to about 40, and a method in which crystallized ZrO2 (dielectric constant=about 40) is used as a dielectric. However, there is a problem of a phenomenon in which the leakage current is increased along with a transition from the amorphous state to the crystalline state.
Moreover, as a dielectric film having a further higher dielectric constant, an STO (SrTiO3) film (dielectric constant=100 to 120), in which the crystal has a perovskite structure, has also been studied. However, there has not yet been obtained a desired precursor having a high vapor pressure as a source of strontium (Sr). Thus, such a film can be formed at a laboratory level, but an advanced film forming technique has not yet been established to a level applicable to mass production.
In addition, since a comparatively good precursor of Ti is commercially available, a TiO2 (dielectric constant=about 80) film has been partly studied. However, similarly to the STO film, there is a problem that TiO2 film has a narrow band gap and a large leakage current.
Further, there have also been studied laminated films of TiO2 and Al2O3 which have structures as shown in FIG. 9, FIG. 10 and FIG. 11.
For example, to form the structure shown in FIG. 9, after lower electrode 901 is formed, dielectric films (Al2O3 layer: 902, TiO2 layer: 903) are formed by an ALD (Atomic Layer Deposition) method. A sequence for forming the dielectric film by the ALD method is shown in FIG. 3. That is, in step 301, an Al precursor is injected into a reaction chamber of an ALD apparatus, and a monolayer film of Al compound is formed. After purging is performed (step 302), an oxidizing agent, such as ozone, is injected into the reaction chamber and a monomolecular layer of aluminum oxide is formed (step 303). Then, the inside of the reaction chamber of the apparatus is purged (step 304). The above described steps are performed as a cycle F for a required number of times, so that aluminum oxide layer 902 having a predetermined film thickness is formed. Similarly, in step 305, a Ti precursor is injected into the reaction chamber, so as to form a monolayer film of Ti compound, and then step 306 to step 308 are performed. These steps are performed as a cycle G for a required number of times, so that titanium oxide layer 903 having a predetermined film thickness is formed. Then, upper electrode 904 is formed. Note that in this specification, a cycle symbol on a loop arrow does not indicate the number of times n of a loop (n is zero or more), but indicates that respective steps surrounded by the loop arrow are successively performed as one cycle. The number of times at which the cycle is successively performed is represented by (n+1) times, that is, the first one loop added to the number n of loops.
Further, in the case where the structure shown in FIG. 10 is formed, after lower electrode 1001 is formed, dielectric films (Al2O3 layer: 1002, TiO2 layer: 1003, Al2O3 layer: 1004) are formed by a film forming sequence of the ALD method as shown in FIG. 4, and then upper electrode 1005 is formed. That is, the inside of the reaction chamber of the ALD apparatus is purged with nitrogen gas in step 401, and an Al precursor is injected into the reaction chamber of the ALD apparatus (step 402), so that a monolayer film of aluminum compound is formed. After purging is performed (step 403), an oxidizing agent, such as ozone, is injected, and a monomolecular layer of aluminum oxide is formed (step 404). Then, the inside of the apparatus is purged (step 405). The above described steps are performed as a cycle H for a required number of times, so that aluminum oxide layer 1002 having a predetermined film thickness is formed. Similarly, a Ti precursor is injected in step 406, so as to form a monolayer film of Ti compound, and then step 407 to step 409 are performed. These steps are performed as a cycle I for a required number of times, so that titanium oxide layer 1003 having a predetermined film thickness is formed. Further, similarly to the cycle H, a cycle H′ for repeating step 410 to step 413 is performed for a required number of times, so that aluminum oxide layer 1004 having a predetermined film thickness is formed. Thereafter, upper electrode 1005 is formed.
Similarly, it is also possible to form the structure (lower electrode: 1101, TiO2 layer: 1102, Al2O3 layer: 1103, TiO2 layer: 1104, upper electrode: 1105) as shown in FIG. 11.
However, the results of the tests performed by the present inventors show that in the structures as shown in FIG. 9, FIG. 10 and FIG. 11, the increase in dielectric constant cannot be sufficiently compatible with the reduction in leakage current as expected.
In addition, the following documents are listed as related techniques.
Patent Document 1 (Japanese Patent Laid-Open No. 2002-373945) discloses a semiconductor device featured by having a capacitance of MIM (Metal-Insulator-Metal) structure which is formed by the ALD method and which uses, as a capacitance insulating film, at least one material selected from ZrO2, HfO2, (Zrx, Hf1-x)O2 (0<x<1), (Zry, Ti1-y)O2 (0<y<1), (Hfz, Ti1-z)O2 (0<z<1), and (Zrk, Til, Hfm)O2 (0<k, l, m<1 and k+l+m=1). Here, (Zrx, Hf1-x)O2 is an oxide of a solid solution of Zr and Hf, and (Zry, Ti1-y)O2 is an oxide of a solid solution of Zr and Ti. Also, (Hfz, Ti1-z)O2 is an oxide of a solid solution of Hf and Ti, and (Zrk, Til, Hfm)O2 is an oxide of a solid solution of Zr, Ti and Hf. Further, the capacitance dielectric film in Patent Document 1 is used by being positively crystallized by annealing.
Patent Document 2 (Japanese Patent Laid-Open No. 2004-274067) discloses a forming method of a high dielectric constant oxide film ((AlxHf1-x)Oy) formed by the ALD method. Here, an Al precursor and an Hf precursor are simultaneously injected into the reaction chamber of the ALD apparatus. Then, a reactant including Al and Hf at a predetermined composition ratio is vapor-deposited, so that Al and Hf are simultaneously oxidized to form (AlxHf1-x)Oy.
Patent Document 3 (National Publication of International Patent Application No. 2004-511909) discloses an integrated circuit in which an interface layer is provided between a conductive material and a dielectric material, and in which the interface layer is selected from a group including an aluminum oxide and a lanthanide oxide and has a film thickness of about four monomolecular layers or less.
In view of the above described problems, it is desired to provide a dielectric film and a method for forming the dielectric film which can be applied to mass production of DRAM capacitor of 60 nm generation or later and in particular to mass production of DRAM capacitor of 45 nm level, and which has a high dielectric constant and a small leakage current.